1. Field of the Invention
This invention relates generally to the fabrication of semiconductor devices. More specifically, the invention pertains to the testing of wire bonds and lead wires, also called bond wires, for integrity.
2. State of the Art
The secure bonding and maintenance of bond wires extending between the bond pads of a semiconductor die and the leads of a corresponding lead frame are critical in the manufacture of semiconductor devices employing this technique. Typically, the method of interconnecting the bond pads of an integrated circuit (IC) device to a lead frame or other carrier having conductive traces comprises individual wire bonding techniques such as thermocompression, thermosonic, or ultrasonic bonding. The wires are typically formed of gold, aluminum, or alloys thereof, and have wire diameters of e.g. 0.001 to 0.003 inch. As wire sizes have become increasingly miniaturized, the inherent strength of the wires and of the wire bonds to the bond pads and leads has necessarily been reduced.
The now-common use of "leads over chip" (LOC) semiconductor die assemblies replaces a traditional lead frame having a central, integral support (commonly called a die-attach tab, paddle, or island) to which the back surface of a semiconductor die is secured, with a lead frame arrangement wherein the dedicated die-attach support is eliminated and at least some of the leads extend over, and are secured to, the active surface of the die. LOC die assemblies may have centrally located bond pads, thus increasing the length of the leads, which may flex during a wire bonding operation.
Because of the high cost of IC devices and the difficulty of correcting defective wire interconnects once the devices are encapsulated, it is vital to achieve a very high degree of reliability during wire bonding. More sophisticated methods and apparatus are required to limit the frequency of wire and wire bond failure.
As well-known in the art, the inadequacy or failure of bond wires and wire bonds may be caused by a wide range of factors, and may result from events occurring (a) at the time of bonding, (b) during assembly steps following bonding but before encapsulation, and (c) during encapsulation.
Inadequately bonded wires may occur because of many reasons, including bond pad surfaces which are not adequately cleaned, incomplete metallization of bond pads, wire impurity, inadequate bonding temperature, stress-strain mismatches, excessive flexing, corrosion, intermetallic grain growth followed by stress-induced creep, as well as by other causes.
The wires themselves may occasionally break because of impurity-induced weakness, corrosion, and mishandling such as an accidental excessive force applied during wire-pull testing.
Wires may also be weakened or fail during die processing steps subsequent to bonding but before encapsulation. Handling of the die during intermediate processing steps such as testing and inspection may result in weakening or separation of inner (to a bond pad) or outer (to a lead or trace) wire bonds as well as occasional breakage of a wire itself.
The step of encapsulating the die with a plastic or ceramic material may also result in wire or bond failure. In transfer molding of a lead frame-mounted semiconductor die, the die is suspended from its active surface from the underside of inner lead extensions of a lead frame (typically Cu or Alloy 42) by a tape, screen print or spin-on dielectric adhesive layer. The bond pads of the die and the inner lead ends of the frame are then conductively connected by wire bonds (typically Au, although Al and other metal alloy wires have also been employed) by means known in the art. When any intermediate steps are completed after wire bonding, the resulting die assembly is placed in a mold cavity and encapsulated in a heated, thermosetting particulate-filled polymer which, upon curing, forms a highly cross-linked matrix no longer capable of being melted. Typically, a post-cure step completes the curing of the polymer. The die and lead frame assembly may comprise the framework of a dual-in-line package (DIP), zig-zag in-line package (ZIP), small outline j-lead package (SOJ), thin small outline package (TSOP), quad flat pack (QFP), plastic leaded chip carrier (PLCC), surface mount device (SMD) or other plastic configuration.
During transfer molding, a defect known as "wire sweep" may become a troublesome problem. In this type of defect, the advancing flow front of liquid thermoset molding compound sweeps the wires against each other, causing short-circuiting. Factors that tend to exacerbate wire sweep include high wire loop heights, long wire bond lengths, bond orientation perpendicular to the advancing polymer flow front, rapid mold compound transfer times, high transfer pressures, rapid viscosity rise of the polymer melt, relatively low wire modulus, and insufficiently bonded wires. The difficulty in precisely controlling all of the above factors results in packaged, yet defective, semiconductor devices which must be discarded at considerable loss.
As is well known in the art, the wires and wire bonds may also fail during or following post-curing because of the stresses formed in the polymer package.
Of course, once the die has been encapsulated by transfer molding, it is very difficult, if even possible, to correct a wire which is broken, shorted, or which has become unbonded from a bond pad or lead finger. Even though the cost of manufacturing a semiconductor device through the encapsulation step is substantial, it is rarely economical to attempt repair of a defective wire or wire bond after transfer molding. Removal of the encapsulant without destroying the interconnecting wires is extremely difficult, particularly when the encapsulant is a transfer-molded filled polymer.
Although the state of the art in wire bonding is continually improving, ever-increasing demands for further miniaturization, increased circuit complexity, enhanced production speed, reduced cost, product uniformity and reliability require further improvements in quality control.
It is apparent that to avoid failure of wires or wire bonds, both the wire modulus and the bond strength should be as high as reasonably possible, given known process parameters. It is thus desirable to provide a method for confirming that prior to encapsulation, all of the wires and/or bonds meet a predetermined minimum value of strength.
One test which is used to determine the wire bond strength comprises the use of a hook to pull a lead wire loop upward with an increasing measured force until a wire bond breaks. This is a destructive test and is not used routinely on production dies.
In a related test, a lead wire is pulled upward by a hook at a minimum threshold force value indicative of satisfactory bonds. Only inadequately bonded leads fail the test, so the test is at least arguably "non-destructive". Wire pull testing is typically conducted under 25.times.-50.times. magnification and is a tedious and time consuming task. Further, damage is occasionally incurred by the wire under test, or by adjacent wires. The bond pulling test is not generally appropriate for testing wire bonds of very closely spaced lead wires used in many recently developed semiconductor devices.
U.S. Pat. No. 3,581,557 of Drees et al. briefly indicates the common problem of inadequate bonding of wires to semiconductor die bond pads and package leads. Drees et al. discloses an apparatus for exerting a "puff" of gas substantially transversely across and at a selected angle to each wire following its bonding to break away weakly bonded wires. The test is conducted on each wire immediately after it has been bonded at both ends, so that any failed wires will not be displaced by the relatively horizontal puff of gas into adjacent wires. Thus, the test must be conducted on a lead wire before the next adjacent wire is bonded to the die and lead frame. If the final wire fails in this test, it may be blown into the adjacent first wire and may displace or break that wire as well. Thus, the test has distinct problems if used following completion of multiple wire bonds. In addition, the "puff" duration is severely limited in order to avoid wire vibration induced by the gas stream.
U.S. Pat. No. 5,085,084 of Salatino discloses a method for testing the bond integrity of lead frame leads to bond pads by applying a fluid to the underside of a plurality of leads simultaneously. A conductive sensor is positioned above the leads, and if a lead-to-bond pad bond fails due to the applied fluid force, its broken end is blown upward to contact the sensor and close an electrical circuit. The Salatino method is also disclosed as being applicable to testing of wire bonds.
Several problems are inherent in the Salatino test. First, the actual force exerted on each lead will depend upon the free area for gas flow in the vicinity of the lead. Thus, the width and proximity of adjacent leads will affect the applied force. Both of these factors may be highly variable, not only from die to die, but in respect to the leads on a single die as well. Further, failure of a single lead-to-bond pad bond results in movement of that lead out of the lead pattern, decreasing the resistance to air flow and reducing the force exerted on adjacent leads. Thus, the force is not evenly applied to the leads.